1. Field of the Invention
The present invention relates generally to rare earth-doped optical amplifiers, and particularly to polymer-based rare earth-doped waveguides that can be used in erbium-doped waveguide amplifiers (EDWAs).
2. Description of Prior Art
Optical amplifiers increase the optical output power of an end-terminal system. They can also be used as repeaters, thus allowing increased distance between end-terminal equipment. Rare earth-doped amplifiers can be stimulated to produce a laser that has the same wavelength as that of incident light. Erbium-doped amplifiers are the most established and accepted rare earth-doped amplifiers.
A type of erbium-doped amplifier called an erbium-doped fiber amplifier (EDFA) is commonly used in transoceanic cable transmission, but is too expensive for use in high-density metropolitan area networks (MANs). Erbium-doped waveguide amplifiers (EDWAs) have many of the advantages of EDFAs, are more economical than EDFAs to use in MAN applications, and yield better price/performance ratios than EDFAs in MANs. An EDWA comprises an erbium-doped waveguide embedded in a glass substrate. The EDWA's similarity to an EDFA derives from its use of an erbium-doped waveguide as a gain medium.
One method for making an EDWA is the ion exchange method, which comprises two steps. In the first step, a glass substrate is prepared in which erbium oxide has been uniformly mixed. The second step is the ion exchange step, in which the glass substrate is immersed in an ion salt bath and the erbium ions are exchanged out of the substrate. Since the outer regions of the substrate are more likely to lose their erbium ions in this process than the inner regions, a density graded distribution of the erbium ions in the substrate results. Thus a tunnel waveguide buried several micrometers (μm) under the glass surface is created. The stability of the waveguide is assured by the glass covering.
Another method for making an EDWA uses sputtering depositions. A target having a suitable composition with regard to the desired composition of the core layer to be deposited is arranged opposite a substrate in a vacuum chamber. Argon and oxygen are then introduced, such that the pressure in the vacuum chamber is in the range of about 0.3 to 25 Pascals (Pa). Radio frequency (RF) power is applied to the target. The target is then bombarded by argon atoms, such that atoms and/or molecules of the target are emitted from the target and deposited on the substrate. This process is continued until the deposited layer has sufficient thickness.
Referring to FIG. 4, U.S. Pat. No. 5,982,973 discloses an erbium-doped planar optical waveguide. The waveguide comprises a substrate, a bottom layer formed on the substrate, an active guiding layer arranged on the bottom layer, and a top cladding layer arranged over the active guiding layer. The sputtering depositions method is used to create the active guiding layer and top cladding layer of the waveguide. The material for the active guiding layer and the top cladding layer is glass.
U.S. Pat. H 1,848 discloses a Z-propagating waveguide laser and amplifier device in which a rare earth-doped LiNbO3 crystal is used as a waveguide substrate. The waveguide is formed in the LiNbO3 crystal substrate, substantially parallel to the crystallographic Z-axis of the LiNbO3 crystal substrate. A metal diffusion method is used to create the graded refractive index of the crystalline waveguide.
Unfortunately, the ion exchange method, the sputtering depositions method of U.S. Pat. No. 5,982,973 and the metal diffusion method of U.S. Pat. H 1,848 are all difficult to control, time-consuming and expensive. An improved method is desired to overcome the above problems in making optical amplifiers.